It is known to use polymer coatings on objects such as complex three-dimensional shapes. Typically, these coatings are produced using thermally activated polymerization. Heat is used to generate active centers. The active centers polymerize the coatings after a monomer has been applied to a substrate as a liquid. However, thermal polymerization requires tremendous amounts of time, energy and expense to run high-temperature ovens for extended periods of time.
It is also known to use photopolymerization to produce polymer coatings on objects. Photopolymerization reactions are chain reactions which generate free radical or cationic active centers. In photopolymerization, energy from UV or visible light is used to polymerize the monomer. Photopolymerization has a number of advantages, including savings in energy and high cure rates without the necessity of solvents.
However, photopolymerization of coatings on complex three-dimensional shapes has not been successful because of problems with oxygen inhibition and shadow regions. Previously known photopolymerization systems have used free radical polymerization to generate free radical active centers. Oxygen inhibition occurs when free radical active centers react with oxygen to produce unreactive peroxides and hydroperoxides. The effect is to decrease the polymerization rate and reduces the polymer molecular weight resulting in an incomplete cure. To overcome this problem, coating systems are purged with nitrogen in an attempt to displace oxygen from the monomer coated surface. While this removes most of the oxygen and allows photopolymerization to occur on flat substrates, it is difficult to purge oxygen from recessed regions, resulting in poor polymerization in these areas.
Problems also occur as the result of shadows from incomplete illumination of the coating material on the substrate during photopolymerization. Typically, photopolymerization is carried out by mixing a monomer, a photoinitiator and any additives to the application. The mixture is then applied on to the substrate, followed by illumination from a light source, to start the photochemical process. Active centers are produced when the photons from the light are absorbed. Therefore, each area of the substrate must be exposed to light. This is difficult to accomplish with complex shapes. Any region not exposed or lying in the shadow will have an absence of active centers. Therefore, the material remains in an uncured state. Multiple illumination sources, moving illumination sources and moving substrate sources have been used in an attempt to overcome this problem.